First Results on Free-form Polishing Using the Precessions Process

1. First Results on Free-form Polishing Using the Precessions Process D. Walker, A. Beaucamp, C. Dunn, R. Freeman, A. Marek, G. McCavana, R. Morton, D. Riley

2. This talk • A word of definition! • Applications of free-form surfaces • Technical approach • Definition of the experiment • Preliminary results • Conclusion

3. A word of definition… • We use ‘free-form’ to include any non-rotationally-symmetric form that has to be produced by a free-form process! • Pragmatic, if nothing else!

4. Free-form in space • More DOF, fewer elements => • lower-mass • greater efficiency • less stray-light • Enable compact all-reflecting designs => James Webb Space Telescope Instrumentation • large wavelength coverage • many applications in earth-observation and astronomical satellites

5. 50m Euro50 50m Euro50 50m Euro50 …and on Earth from…. • Next generation of 30-50 metre ground-based telescopes, with hundreds of off-axis aspheric segments • Precision engineering components such as stainless steel prosthetic joints, moulds etc. Euro50 50m telescope to… Knee joint

6. The experiment • Aim to remove material according to a pre-defined complex form • Separate the challenges of form control, CNC and machine-kinematics, from metrology issues • Produce the form on: • a flat surface • a concave spherical surface

7. The Zeeko-ClassicTM Process • Uses an inflated, spinning, precessed ‘bonnet’ • Covered with standard polishing cloth • Operates with standard polishing slurry • Creates a near-Gaussian influence function • Can start with precision-ground surface • 7-axis CNC machine-tool • Gives all DOF to maintain constant geometry between tool and all points on a free-form surface • Form-control by numerical-optimisation

8. 200mm Zeeko machines Prototype used for the free-form experiment Zeeko-Loh AII

9. Method used • Polish and measure an influence function (8mm Ø) • Define the parent surface in Rhinoceros CAD • Define the required 3D feature to be polished into the substrate, using Rhinoceros CAD • Input to optimizer through NURBS interface • Numerically-optimise the dwell-time map to achieve the required 3D removal • Convert to raster tool-path with variable speed • Polish (24 mins) and measure the result

10. Target material removal (‘error-map’) The designed feature 0.5 μm deep land + another 0.5 μm deep logo

11. Screen-shot (flat substrate) Initial profile (flat in this case) Influence function Error map (target removal) Predicted material removal Optimised dwell-time map Predicted residuals

12. Zygo result on flat

13. Analysis of flat result • Confirmed the correct: • dwell-time prediction of the optimiser • generation of the tool-path • execution by the machine • Other than … a ~0.5μm wedge on the 0.5μm deep land • found to be due to a sign-error in the wedge-compensation code – then corrected!

14. Screen-shot (concavesubstrate)

15. Zygo result on concave

16. Polishing result as seen in the Zeeko Optimiser

17. Analysis of concave result • Wedge error was removed • 3D form correctly reproduced • Absolute material depth correctly predicted at ~80% level. • Beyond the flat result • confirmed correct 3D kinematics of machine, including precessing about the local normal at all points over the surface

18. And finally… • Extensive trials conducted polishing truly free-form surfaces (precision stainless-steel lugs) • Form preserved and texture improved Ra ~ 170nm Ra ~ 18nm

19. Conclusion, and the future • We have demonstrated the core operation of a highly-versatile free-form corrective polishing process! • We are about to correct form on truly free-form lenses – all software now in place • Metrology of choice is the 200mm traverse-length Form Talysurf 1240, with integrated Y-stage and 2μm diamond stylus. • We have had excellent results using the latest software and with correct calibration procedures

20. Form Talysurf error-map of truly free-form surface to be produced next. – wait for SPIE Denver!

21. Thank you!